One class of device is that which uses an organic light emitting device, or “OLED”, or as the active component of a photocell or photodetector (a “photovoltaic” device). The general structure which is utilised for this type of device is an organic layer with semiconducting characteristics which is provided on opposing planar surfaces with, on one side a cathode for injecting or accepting negative charge carriers (electrons) and on the other side with an anode for injecting or accepting positive charge carriers (holes) into the organic layer.
In organic electroluminescent devices, electrons and holes are injected into the organic layer. The electrons and holes combine to generate excitons that undergo radiative decay. This is disclosed, for example in WO 90/13148, where the organic light-emissive material is a polymer, namely poly(p-phenylenevinylene) (“PPV”). Other light emitting polymers known in the art include polyfluorenes and polyphenylenes. In U.S. Pat. No. 4,539,507 the organic light-emissive material is of the class known as small molecule materials, such as (8-hydroxyquinoline) aluminium (“Alq3”). WO 99/21935 discloses the class of materials known as dendrimers.
In most devices which are capable of practical implementation at least one of the electrodes is transparent to allow photons to escape the device.
Organic photovoltaic devices have the same construction as an organic electroluminescent device, with the exception that charge is separated rather than combined. An example of a photovoltaic device is described in, for example, WO 96/16449.
In known forms the OLED can be produced on a glass or plastic substrate coated with a transparent electrically conducting first electrode such as indium-tin-oxide (“ITO”). A layer of a thin film of at least one electroluminescent organic material covers the first electrode followed by the layer of electroluminescent organic material and a cathode layer is applied to cover the layer of electroluminescent organic material.
Conventional devices can now be described for a conventional polymer electroluminescent device (it will be appreciated that non-polymeric electroluminescent materials may equally be used in place of the polymeric electroluminescent materials described below). The standard architecture of the device includes a transparent glass or plastic substrate, an anode of indium tin oxide and a cathode. An electroluminescent polymer layer is located between the anode and the cathode. If the cathode was applied using a conventional sputtering operation a barrier layer is required to be provided to protect the polymer from damage. The electroluminescent polymer can be present alone or as a plurality of polymers. Where a plurality of polymers are deposited, they may comprise a blend of the electroluminescent polymer with at least one of a hole transporting material and an electron transporting material as disclosed in WO 99/48160. Alternatively, the electroluminescent polymer layer can be formed from a single, block copolymer that comprises regions selected from two or more of hole transporting regions, electron transporting regions and emissive regions as disclosed in, for example, WO 00/55927 and U.S. Pat. No. 6,353,083. Each of the functions of hole transport, electron transport and emission may be provided by separate polymers or separate regions of a single polymer. Alternatively, more than one function may be performed by a single region or polymer. In particular, a single polymer or region may be capable of both charge transport and emission. Each region may comprise a single repeat unit, e.g. a triarylamine repeat unit may be a hole transporting region. Alternatively, each region may be a chain of repeat units, such as a chain of polyfluorene units as an electron transporting region. The different regions within such a polymer may be provided along the polymer backbone, as per U.S. Pat. No. 6,353,083, or as groups pendant from the polymer backbone as per WO 01/62869 .
Further layers may be provided between the anode and the cathode. For example, a hole injection layer such as poly(ethylene dioxythiophene)/polystyrene sulfonate (PEDOT-PSS) as disclosed in EP 0901176 and EP 0947123 or polyaniline as disclosed in U.S. Pat. No. 5,723,873 and U.S. Pat. No. 5,798,170 may be provided between the anode and the electroluminescent material. Further layers that may be present include layer for transporting charge (i.e. holes or electrons); layers for blocking charge; and exciton blocking layers.
For ease of processing, it is preferred that the polymer(s) used in the device are soluble. Substituents such as C1-10 alkyl or C1-10 alkoxy may usefully be selected to confer on the polymer solubility in a particular solvent system. Typical solvents include mono- or poly-alkylated benzenes such as toluene and xylene or THF. Preferred solution processing techniques for deposition of polymers include spin-coating for unpatterned devices or simple segmented displays and inkjet printing for high resolution displays, in particular full colour displays. Inkjet printed, full colour devices may be made by inkjet printing red, green and blue electroluminescent polymers into wells (formed by standard photolithographical techniques) to form corresponding red, green and blue subpixels. Inkjet printing of electroluminescent materials is described in more detail in EP 0880303.
The cathode includes a layer comprising a material that has a workfunction allowing injection of electrons into the electroluminescent layer. The cathode may consist of a single material such as a layer of aluminium, however it is preferred that the cathode includes a material having a workfunction of less than 3.5 eV, more preferably less than 3.0 eV. For example, it may comprise a plurality of metals, for example a bilayer of calcium and aluminium as disclosed in WO 98/10621 or elemental barium disclosed in WO 98/57381, Appl. Phys. Lett. 2002, 81 (4) , 634 and WO 02/84759. Alternatively or additionally, the cathode may comprise a thin layer of dielectric material to assist electron injection, for example lithium fluoride disclosed in WO 00/48258 or barium fluoride, disclosed in Appl. Phys . Lett. 2001 , 79(5) , 2001.
The device will typically suffer degradation if exposed to atmospheric moisture or oxygen. The device is therefore preferably sealed from the environment using a suitable encapsulant.
If light is emitted through a transparent substrate then the device may be encapsulated using a metal enclosure having a cavity into which the device is received and a perimeter that is glued to the substrate to form an airtight seal.
If light is emitted through a transparent cathode then a transparent encapsulant is required. Suitable transparent encapsulants include a sheet of glass located over the device and glued to the substrate or alternating layers of a polymer and a ceramic forming a transparent multilayer barrier having a tortuous path for ingress of moisture or oxygen.
Electroluminescent devices may be monochrome, multicolour or full colour devices.
There are two basic types of OLED.
A first form is referred to as a bottom emitter OLED. This form allows light to pass through the transparent ITO anode and the substrate. This form of device normally has a cathode including a layer of low workfunction metal formed by thermal or e-beam evaporation which does not cause any significant damage to the underlying organic material. In one embodiment this layer of low workfunction metal can be a bilayer of barium with an external coating of aluminium to protect the reactive barium layer.
A second form of OLED is that which includes control circuitry associated with each pixel of the OLED and located underneath the emissive material and is referred to as an active matrix OLED. The control circuitry is not transmissive which means that the total area available for bottom emission (so-called aperture ratio) is reduced and can adversely affect the capabilities and range of uses of this form of device. As a result an OLED device which allows emission through the top of the device is desirable.
It is preferred that the same cathode is used for top emitters as bottom emitters, however bottom emitter cathodes are typically applied at a thickness that make them opaque. Although suitable metals may be transparent at very low thickness, their lateral conductivity at such thicknesses is poor. Also, if a reactive metal is used such as calcium or barium then it needs to be capped with a protective layer. There are relatively few conductive materials that retain transparency at higher thicknesses. One such suitable class of materials are transparent conducting oxides (TCOs), for example indium tin oxide (ITO) and doped Zinc Oxide (ZnO), for example indium zinc oxide (IZO).
The problem with ITO and other materials of this type is how to apply the same onto the device in a manner which allows the required adhesion and also prevents damage being caused to the material onto which the ITO or similar material is being applied. It is known that ITO is not amenable to evaporation and so attempts have been made previously to sputter the ITO and/or use barrier layers which are first applied in order to protect against damage being caused by the sputter process on the material layer. However the need to use barrier layers is generally undesirable but has to date been regarded as being necessary in order to avoid the damage caused by sputtering. An aim of the invention is to minimise the thickness of the barrier layer or entirely eliminate the need for a barrier layer.
One known form of sputtering is where a series of magnetrons are provided in a coating chamber in which the substrates to be coated are held or are positioned adjacent thereto. The magnetrons include a target of the material which is to be deposited by sputtering. The magnetrons are provided in an arrangement which can be referred to as a closed field in which adjacent or opposite magnetrons and/or magnet arrays are arranged, typically with reverse polarity of the respective outer magnets between adjacent magnetrons and/or magnet arrays, so as to provide a field within which the plasma which is generated is retained or “trapped” and is relatively dense while producing sputtering of the material from the targets at relatively high rates of deposition. Furthermore the magnetrons are typically unbalanced. The devices or substrates to be coated can be biased, for example, by the application of a voltage thereto, or held at a floating potential. An example of a closed field system is disclosed in the patent GB2258343.